The Believers were told categorically by the Almighty Allah, i.e. the One in Whom they believe that all of them were a single brotherhood and that He has named them Al-Muslimeen. But the so-called believers would not like the name Al-Mulsimeen as given to them by Allah but later on would like be known by other names, e.g., Sunni, Shia, Ahl-e-hadith, Ahl-e-fiqh, Ahl-e-Quran, Hanafi, Maliki, Hambali, Shafi’ee, Deobandi, Barailivi, Naqshbandi, Chishti, Suhrwardi, and so on. Perhaps this was not enough and they divided themselves even further into other divisions based on political parties, ethnicity, language, geography, and so on.
How could there be differences among them, had they been sticking to the Book revealed by the Almighty Who claimed that the very Book had no contradictions and differences in it. Had it been from anyone other than Allah, there may have been discrepancies or ikhtilaaf in it. How could this book cause any differences when the very purpose of this book was to clarify and resolve all the differences. The Almighty had enjoined upon the believers to hold Allah’s rope (i.e. His Book) firmly all together, establish Ad-Deen (i.e. Allah’s Law) and do not become divided.
The Almighty had warned them in His Book that dividing Ad-Deen into various sects is like associating partners with Allah and doing Shirk which is unforgivable (since in doing so one takes humans as the final authority instead of Allah and His Book). The Almighty also made it abundantly clear that sectarianism and division is worse even than worshipping a calf due to ignorance. Remember! How the Prophet Aaron (a.s.) justified to the Prophet Moses (a.s.), worshiping of the calf by the Children of Israel for some time to avoid divisions among them in the longer run.
The Almighty had further warned the Believers that if they divide themselves into sects, then Allah’s Prophet (s.a.w.) will have no concern whatsoever with them. He further explained with an example that even a holy place like a Masjid that causes division among the believers is unacceptable and needs to be destroyed.
In spite of all these warnings and explanations, there still have been many who either created sects or led sects or followed sects, mostly not because they were deficient in knowledge but because of selfish rivalry and jealous animosity among one another.
After ignoring so much explanation and so many warnings by the Almighty, the punishment was inevitable. A part of this punishment they are suffering in this world in the form of internal strife, conflicts and killings, a part by external attacks, and a part will certainly they suffer in the hereafter.
 Al-Quran Surah 49: Verse 10
 Al-Quran Surah 22: Verse 78
 Al-Quran Surah 4: Verse 82
 Al-Quran Surah 16: Verse 64
 Al-Quran Surah 3: Verse 103, Surah 42: Verse 13
 Al-Quran Surah 30: Verses 31-32; Surah 4: Verse 48
 Al-Quran Surah 20: Verses 88-94
 Al-Quran Surah 6: Verse 159
 Al-Quran Surah 9: Verses 107-108
 Al-Quran Surah 42: Verse 14; Surah 2: Verse 213; Surah 3: Verse 19; Surah 45: Verse 17
 Al-Quran Surah 3: Verse 105